1. ## Large Electrons

There is a hydrogen atom in a vacuum; I cannot think of anything that would slow down the electron, so I guess it should get close to the speed of light. Somewhere I read that as matter approaches the speed of light, the mass increases to infinity. I am clearly missing something here since the only electrons I know of are really small.

2. Your problem is kind of the opposite thinking from the mistake of the ancient Greeks, but still a mistake. The ancient Greeks thought that in a vacuum, moving objects would come to a halt, because not being stationary required some kind of influence. You know that's wrong, but you've gone too far-- you imagine that in a vacuum, a particle will continue to speed up rather than slow down. Galileo discovered that actually, what you should expect in a vacuum, and with no outside influences, is that the particle should maintain whatever speed it had before. For an electron in a hydrogen atom, that speed is about 1% of the speed of light, and has no reason to ever increase.

3. What stops the electron from going faster than 1%?

4. Consider this: what influences the electron to go faster than 1%?

I assume you mean when the electron moves into a location that is a vacuum. Please correct me if I'm wrong in that assumption.

5. I mean when it is orbiting around the nucleus, I just thought of something, would it no longer be in orbit if it went any faster (assuming the orbit is a circle)?

6. I'm sure Ken could answer that definitively, but I would expect what you're saying there is correct, that it couldn't go any faster without carrying too much energy for that orbit level.

I'm still unsure though, what do you propose would be the reason for the electron wanting to increase it's speed in the first place? I'm somewhat confused by that.

(ETA - Your mention of the electron's mass increasing to infinity as it speeds up, and the size of the electron, this seems confused to me. I don't believe the item grows in size as it speeds up, but rather in mass density due to the energy it's carrying.)

7. Originally Posted by VARN
I mean when it is orbiting around the nucleus, I just thought of something, would it no longer be in orbit if it went any faster (assuming the orbit is a circle)?
I think it was once believed that electrons "orbited" the nucleus like planets around the sun, but that is no longer seen as the reality. I don't really understand, but electrons don't actually inhabit a certain spot in an orbit. It's more like, they're statistically distributed around the shell. So the electron has a certain likelihood of being found at some place, but it isn't really orbiting.

But even if it were, why would it go faster and faster? The earth has been orbiting the sun for "billions and billions" of years, and it doesn't speed up.

8. It is not that it wants to increase the speed; it is that there is nothing to slow it down. If you had a proton and an electron, placed them close to one another in a vacuum, the electron would start to orbit around the proton; since at first it was at rest, to get to 1% the speed of light it must accelerate. What stops the acceleration?

Using the word orbit to simplify the movement.

9. I've never heard that suggestion before as you put it. Depending on the initial conditions of the proton and electron, so how far apart from each other and their momentums, they may end up orbiting each other, but unless energy is added or removed I fail to see why they should change their speed of movement in the way you describe. I certainly don't see why they should tend towards a specified speed.

I'd be interested to hear more from a more knowledgable person, as I'm applying fairly limited knowledge here really.

ETA: unless they were really close together I wouldn't expect them to necessarily orbit each other.

10. Originally Posted by VARN
It is not that it wants to increase the speed; it is that there is nothing to slow it down.
Yes, but even if there is nothing to slow something down, it doesn't go any faster. As I pointed out, the earth has been orbiting the sun for billions of years, with nothing to slow it down, and yet it doesn't speed up. Right?

11. Right-- the Earth needs a reason to speed up just as much as it needs a reason to slow down. It's true that the Sun's gravity could allow the Earth to speed up, but it still doesn't, because it doesn't have a reason to-- the Earth is happy in its current orbit, and something has to change to change that. This was Newton's core message-- to understand motion, you have to understand the influences that change motion. Otherwise you should expect whatever motion was happening before (whether motion in a straight line or in an orbit) to just continue.

(In an atom, there is an additional constraint on changes in motion-- the motion cannot be confined to too small a region unless you can supply a huge amount of energy, because it turns out that highly confined motion is highly energetic motion, in quantum mechanics. Yes that is counterintuitive, as there is no classical analog. Now, the electric field of a proton can supply some energy, but not enough to confine the electron motion to a region much smaller than 1 Angstrom across.)

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Originally Posted by VARN
It is not that it wants to increase the speed; it is that there is nothing to slow it down. If you had a proton and an electron, placed them close to one another in a vacuum, the electron would start to orbit around the proton; since at first it was at rest, to get to 1% the speed of light it must accelerate. What stops the acceleration?

Using the word orbit to simplify the movement.
The electron does not move around the nucleus no matter what word you want to use for it. You cannot track any sort of path that the electron follows.

Oddly enough, even tho you cannot sensably make a trajectory for an electron, I can tell you why it would get up to a momentum. The energy in this case is the electrostatic attraction between proton and electron. What stops the acceleration is angular momentum and the uncertainty principle.

Sort of.

This is a very oversimplified description. To say that an electron moves in an atom is just plain wrong

13. Correct me if I'm wrong here but a free electron when it is captured by a nucleus of an atom will release its kinetic energy in the form of a photon will it not? The wavelength of said photon is a function of the electrons kinetic energy and the shell of the atom is settles into. Bear in mind it may drop any number of shells and release further photons until it reaches its ground state and the number of photons is not deterministic but the total energy of all the photons are and possible combinations of photons with their respective individual energies is calculable.

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Saying what Wayne just said in slightly different words,

If an electron has more energy than it needs to be captured into a
particular orbital of an atom, it can release that energy as a photon.
It can release another photon to drop into a less-energetic orbital,
until the electron is in the ground state, where it cannot release any
more energy.

Electrons do move inside atoms. They do orbit atomic nuclei.
But they do not orbit in any way that has a simple description,
and, more importantly, they do not move in a way such that
their motions can be tracked. It is fundamentally impossible
to track the motions of an electron inside an atom.

The Rutherford model of atoms, which is what most people think
of today when they try to visualize atoms, was current for a few
months in 1911. Almost 100 years ago!

-- Jeff, in Minneapolis

15. Originally Posted by WayneFrancis
Correct me if I'm wrong here but a free electron when it is captured by a nucleus of an atom will release its kinetic energy in the form of a photon will it not?
Not quite-- it does have to release energy (as a photon or in other ways) to be captured, but it will still end up with even more kinetic energy after it enters the atom than it started with. The same would be true classically, say for a moon being captured by a planet.

16. Originally Posted by Ken G
Not quite-- it does have to release energy (as a photon or in other ways) to be captured, but it will still end up with even more kinetic energy after it enters the atom than it started with. The same would be true classically, say for a moon being captured by a planet.
What you are saying doesn't make sense....lets take a the following example. We hit the electron with a high energy photon. Said photon is absorbed by the electron and kicked out of the atom because the energy was enough to do so.

This electron now is zipping around with that extra energy. If it pops into another atom it must give up that kinetic energy.

If we take the inverse of what you are saying then an electron must lose energy to be stripped from the atom which doesn't make sense.

17. Originally Posted by WayneFrancis
What you are saying doesn't make sense....lets take a the following example. We hit the electron with a high energy photon. Said photon is absorbed by the electron and kicked out of the atom because the energy was enough to do so.

This electron now is zipping around with that extra energy.
If it got a really big hit, that's true, but more typically, it will end up with less kinetic energy than it started. Or if it is not knocked clear out of the atom, but only excited to a higher energy level, then it will also have less kinetic energy. Again, the same is true classically, this is not anything fundamentally quantum in nature.

If it pops into another atom it must give up that kinetic energy.
Yes, but it usually gets even more from the new atom.
If we take the inverse of what you are saying then an electron must lose energy to be stripped from the atom which doesn't make sense.
It's counterintuitive at first, but it does make sense. For example, Pluto has less kinetic energy per gram than does the Earth, yet to get the Earth into Pluto's orbit, we'd have to add energy, not subtract it. The difference is made up in gravitational potential.

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VARN,

There is a hydrogen atom in a vacuum; I cannot think of anything that would slow down the electron, so I guess it should get close to the speed of light. Somewhere I read that as matter approaches the speed of light, the mass increases to infinity. I am clearly missing something here since the only electrons I know of are really small.
There is a hydrogen atom in a vacuum; I cannot think of anything that would slow down the electron, so I guess it should get close to the speed of light. Somewhere I read that as matter approaches the speed of light, the mass increases to infinity. I am clearly missing something here since the only electrons I know of are really small.
In a hydrogen atom the electron orbital speed is thought to be roughly 1/150 the speed of light. For the innermost shell of heavy elements like uranium, electrons orbit closer to the speed of light. Electrons (shells) that are at greater distances from the nucleus have slower orbital velocities and less momentum. The larger the atom the closer the inner electrons are to the nucleus.

Large electrons
Heavy electrons are theoretical but there is evidence for their existence

http://www.lenr-canr.org/acrobat/Ale...heavyelect.pdf

Muons are short-lived relatively large negative electron-like particles about 200 times the mass of an electron.
They orbit closer to the nucleus but not as fast as an electron would at the same distance.

http://en.wikipedia.org/wiki/Muon
Last edited by forrest noble; 2010-Sep-08 at 01:59 AM.

19. Originally Posted by Ken G
If it got a really big hit, that's true, but more typically, it will end up with less kinetic energy than it started. Or if it is not knocked clear out of the atom, but only excited to a higher energy level, then it will also have less kinetic energy. Again, the same is true classically, this is not anything fundamentally quantum in nature.

Yes, but it usually gets even more from the new atom.
It's counterintuitive at first, but it does make sense. For example, Pluto has less kinetic energy per gram than does the Earth, yet to get the Earth into Pluto's orbit, we'd have to add energy, not subtract it. The difference is made up in gravitational potential.
Ok I've clearly got some more learning to do on electron shells, atomic orbital and ionization energy.

I understand the gravitational potential situation but as I understand it atoms work very differently. In gravity the planets closest to the centre have the highest kinetic energy, ie move the fastest. While in an atom electrons in the outer atomic orbitals move faster.

Is the process you are talking about related to electron capture and beta decay?

20. Originally Posted by WayneFrancis
I understand the gravitational potential situation but as I understand it atoms work very differently. In gravity the planets closest to the centre have the highest kinetic energy, ie move the fastest. While in an atom electrons in the outer atomic orbitals move faster.
No, the opposite is true-- it's just like the classical situation. What makes an atom weird is nothing to do with the amount of energy, it has to do with the discrete nature of the energy-- in particular, there is a minimum energy below which it does not go, and that limits how much kinetic energy the electron can get. A planet, on the other hand, does not have that minimum energy, so could have much higher kinetic energy per gram than Earth, be much closer to the Sun, and have a much lower total energy when the potential energy is included.
Is the process you are talking about related to electron capture and beta decay?
No, it would be true in a completely classical treatment of the electron orbitals, if we simply impose the discrete energies. Nothing really very quantum about it, nor reliant on any unusual physics. It might help to google the "virial theorem."

21. Originally Posted by Ken G
No, the opposite is true-- it's just like the classical situation. What makes an atom weird is nothing to do with the amount of energy, it has to do with the discrete nature of the energy-- in particular, there is a minimum energy below which it does not go, and that limits how much kinetic energy the electron can get. A planet, on the other hand, does not have that minimum energy, so could have much higher kinetic energy per gram than Earth, be much closer to the Sun, and have a much lower total energy when the potential energy is included.
No, it would be true in a completely classical treatment of the electron orbitals, if we simply impose the discrete energies. Nothing really very quantum about it, nor reliant on any unusual physics. It might help to google the "virial theorem."
Thanks

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About one minute elapsed between wondering how Wayne got the
idea that "electrons in the outer atomic orbitals move faster", and
re-reading in my own post above that an electron "can release
another photon to drop into a less-energetic orbital". Clearly, it
looks as if an electron in a higher-energy orbital should be moving
faster, but of course it isn't.

When I say "higher-energy", what do I mean? Energy is relative.
What is the higher-energy orbital higher in energy relative to?

Pluto is in a higher-energy orbit than Earth, because it is farther
from the Sun. But it is also therefore moving slower than Earth,
so has less kinetic energy per gram.

So the energy involved in the electrical attraction between an
electron and an atomic nucleus works the same way as energy
in gravitational attraction between the Sun and planets. Is it
electric potential energy that an electron has more of in higher-
energy orbitals?

-- Jeff, in Minneapolis

23. Originally Posted by Jeff Root
Is it
electric potential energy that an electron has more of in higher-
energy orbitals?
Yes.

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Originally Posted by Jeff Root
Saying what Wayne just said in slightly different words,

If an electron has more energy than it needs to be captured into a
particular orbital of an atom, it can release that energy as a photon.
It can release another photon to drop into a less-energetic orbital,
until the electron is in the ground state, where it cannot release any
more energy.

Electrons do move inside atoms. They do orbit atomic nuclei.
But they do not orbit in any way that has a simple description,
and, more importantly, they do not move in a way such that
their motions can be tracked. It is fundamentally impossible
to track the motions of an electron inside an atom.

The Rutherford model of atoms, which is what most people think
of today when they try to visualize atoms, was current for a few
months in 1911. Almost 100 years ago!

-- Jeff, in Minneapolis
Is the Rutherford model of atoms the same as the Bohr model of atoms?

Is a free electron like a particle and when it is captured by a nucleus then it becomes cloudlike?

Or does it move so fast around the nucleus that it just appears to be everywhere at once?

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Rutherford found that the large majority of alpha particles shot at
a thin gold foil went straight through without being deflected at all,
but a few bounced back at all angles. He interpreted that to mean
that a large electric charge and most of the mass of an atom was
concentrated in a very small volume inside the atom.

That suggested an arrangement very much like the Sun orbited by
planets, so Rutherford's model is also called the planetary model
of the atom.

Bohr built on Rutherford's model, explaining the spectral emission
lines observed to come from atoms as quantum limitations on the
possible orbits electrons could have inside atoms.

The modern theory has many additional aspects which take into
account Heisenberg's uncertainty principle and matrix mechanics,
Schrödinger's wave mechanics, the Pauli exclusion principle, and
the summation of all this, quantum electrodynamics, developed
by Feynman and others in the 1940's. Then the nucleus was
further modeled in quantum chromodynamics by Gell-Mann and
others in the 1950's and 1960's. And the models are still being
refined. Chemistry and "nuclear chemistry" are exceedingly
complex subjects. They basically elaborate all the details of
how electrons and nucleons in atoms can interact.

I'll go along with the idea that a free electron is like a particle
and when it is captured by a nucleus then it becomes cloudlike.
It seems like a good description.

I think electrons in atoms are cloudlike principally because they
are so small. The Heisenberg uncertainty principle describes
how precisely the position and momentum of anything can be
known. Because of their tiny mass, the uncertainty in position
of an electron in an atom is relatively large -- comparable to the
size of the atom.

-- Jeff, in Minneapolis

26. I wonder if VARN is satisfied with the answers? He/she seems to be a member who visits once every year or so, so who knows when he/she will be back.

27. I am satisfied with the answers, I am still reading about the angular momentum and muons; I visit frequently but rarely post.

28. Originally Posted by VARN
I am satisfied with the answers, I am still reading about the angular momentum and muons; I visit frequently but rarely post.
Ah, I should have considered that. Being the loudmouth type myself, I forgot to consider there are people who visit but don't post. Actually, I wasn't really posting so much about you, but I was thinking that we might be straying a bit from the question that you originally asked. But I suppose it all ties in.

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